1. Field of the Invention
This invention relates to the recovery of petroleum from a subterranean formation utilizing wells for injecting heated fluids into the formation, for passing electrical current through the formation, and for withdrawing petroleum from the formation.
2. Description of the Prior Art
Among the more promising methods that have been suggested or tried for the recovery of oil from viscous oil reservoirs are those which introduce thermal energy into the reservoirs. The viscosity of the oil in these reservoirs is generally so high that the oil cannot be recovered at economical rates using conventional techniques. However, the viscosity of these oils can generally be radically reduced by heating. Consequently, when thermal energy is introduced into these reservoirs and the oil is heated, the viscosity will generally be reduced to a point that the oil can be made to flow at efficient and economical rates.
The thermal energy may be in a variety of forms. Hot water flooding, in situ combustion, electric heating, and steam injection are examples of methods using thermal energy that have been used to recover oil from these viscous oil reservoirs. Each of these thermal methods can be useful under certain circumstances. However, hot water flooding and in situ combustion are not widely used in recovering highly viscous oils. In such applications both methods have proven to be deficient in certain respects. Also, as will be discussed later in more detail, electrical and steam heating have been found to have undesirable drawbacks under certain circumstances.
Steam flooding, also known as steam displacement or steam drive, has been used successfully to recover low and medium viscosity crude oils from subterranean formations. In this process, steam is continuously injected into a pattern of injection wells to displace oil to interspersed producing wells. Application of this process to formations containing highly viscous crude oils, however, often requires unachievable pressure gradients or extremely close, uneconomical well spacing. It is desirable, therefore, to preheat the formation by some means prior to the initiation of a steam flood and thereby reduce the oil viscosity to a level that will permit displacement with reasonable pressure gradients and well spacings.
One method for preheating the formation involves cyclic steam stimulation of all wells in a pattern for several years. Heat transfer by conduction and convection causes the heated volume around each well to increase with successive cycles until heat connection between the wells is achieved. Subsequently, the steam stimulation pattern can be converted to a steam drive process with steam being injected into one row of injection wells and oil withdrawn from a row of offset producing wells. Advantages of this method include relative simplicity and significant oil production during the preheating phase. A major shortcoming of this method is that the rate of advance of the heat front decreases significantly as the heated area increases. Also in reservoirs containing highly viscous crudes, it may be necessary to inject steam at pressures in excess of the parting or fracturing pressure for the formation to provide a path for steam entry. This will cause the heated area to assume an elliptical configuration with the major axis oriented in the direction of the parting or fracturing trend for the area. Heat transfer normal to this preferred flow trend may be quite slow and hence it might be quite difficult to heat the area between adjacent wells in a direction normal to this preferred flow trend.
Efforts to overcome this problem have included preliminary injection of a relatively small volume of steam at a pressure which is less than the formation breakdown pressure. Subsequently a second and larger volume of steam is injected into the formation at a pressure greater than the formation breakdown pressure. The first injection step creates a heated zone which is substantially cylindrical. The second injection step generally creates a fracture or pressure parting of the formation which is highly conductive to injected fluids. As a consequence, the heated region around the injection well is larger and it remains approximately cylindrical (U.S. Pat. No. 3,739,852, Woods et al.). While this method is promising, it does not overcome the problem of large areas of unheated oil existing between adjacent rows or producers. Hence, subsequent steam flooding may not be successful.
A second proposed method for preheating a formatioon involves completing wells as electrodes and passing a current through the formation thus increasing the temperature through resistive heating (U.S. Pat. No. 2,801,090 Hoyer et al.). The principal advantage of this method is that electrical current can flow where fluid flow is difficult or impossible. Although this method shows promise, it suffers from some disadvantages. There is no significant oil production during the preheating phase which is unfavorable from an economic standpoint. Also, geometric effects result in very high current densities near the electrode well which results in excessive power losses and temperatures near the well. These very high current densities occur near the well since the well is, in effect, a relatively small-diameter electrode compared to the size of the reservoir being heated. Temperatures near the electrode very rapidly reach steam temperature for the existing reservoir pressure and this causes the connate water within the reservoir to begin to vaporize. The resultant gas saturation causes a drastic increase in formation resistivity since the conductive flow path for electrical current -- connate water -- is removed and this effectively blocks further power input into the formation. Thus, after a short time, the oil in the regions between electrode wells cannot be effectively heated.